Uber's licence to operate in London has not been renewed over safety and security concerns, Transport for London (TfL) said.
A spokesman for the transport body said "several breaches that placed passengers and their safety at risk" were identified.
The ride-hailing app firm's existing licence expires at 11.59pm on Monday, but it will be allowed to continue to operate pending an appeal.
TfL's director of licensing, regulation and charging Helen Chapman said: "As the regulator of private hire services in London we are required to make a decision today on whether Uber is fit and proper to hold a licence.
"Safety is our absolute top priority. While we recognise Uber has made improvements, it is unacceptable that Uber has allowed passengers to get into minicabs with drivers who are potentially unlicensed and uninsured.
"It is clearly concerning that these issues arose, but it is also concerning that we cannot be confident that similar issues won't happen again in future."
